It is a well-known fact that the pace of pumping the air into a tire is directly proportional to the inner diameter of the cylinder of an air pump. However, it is difficult to continue pumping the air into the tire with such an air pump as described above after the air pressure in the tire has reached a certain level. Such a difficulty can be overcome by making the inner diameter of the air pump cylinder smaller at the expense of the pace at which the air is pumped into the tire.
This inventor of the present invention discloses in the Taiwanese Patent Serial No. 79207439 a tire pumping device, which comprises an air guiding tube having one end that is fastened to the head of the pump. The head is provided with a connection head engageable with an air valve of the tire. Another end of the air guiding tube is fastened with a two-way air admitting piston to which a cylinder is pivoted. The cylinder is provided respectively at both ends thereof with a one-way air admitting valve. The air pumping action is carried out at a relatively fast pace by causing the cylinder to move back and forth.
Another improved air pump is disclosed by this inventor of the present invention in the Taiwanese Patent Serial No. 80210881. The air pump is provided with a second cylinder having one end fastened to the head of the pump and having an inner diameter greater than that of the first cylinder. One end of the first cylinder is received in the second cylinder and provided with a one-way pumping valve. The fastening end of the second cylinder is provided with a one-way valve. As the first cylinder is pushed into the second cylinder, the air inside the two cylinders is Compressed simultaneously and is then caused to move toward the head. The air pumping action is carried out at a rather fast pace by the first cylinder alone. However, the air pumping action can not be carried on further without difficulty after the tire pressure has reached a certain level.